I found a picture that resembles that pretty well. It’s called crystalline efflorescence, which is different than the powdery stuff I am used to.
Efflorescence can look very similar to white mold growth. It occurs when moisture moves through concrete (or other types of masonry) and pulls minerals along with the water. As the water evaporates at the surface, the salts and minerals are left behind, leaving a crystalline growth.
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